The present invention relates to printing onto rigid substrates, and to the printing onto textured, contoured or other three-dimensional substrates. The invention is particularly related to the printing onto such substrates as those having textile fabric surfaces or molded objects, rigid panels such as office partitions, automobile interior panels and other contoured objects, and to such printing using ink jet printing techniques.
Applying ink to a substrate by ink jet printing requires a proper spacing between the ink jet nozzles and the surface of the substrate to which the printing is applied. Normally, this spacing must be set to within one or two millimeters to maintain effective printing by an ink jet process. If the distance from the nozzles to the surface being printed is too great, deviations from ideal parallel paths of the drops from different nozzles become magnified. Further, the longer the flight path of the drops from the printhead to the substrate, the more dependent the accuracy of the printing becomes on the relative speed between the printhead and the substrate. This dependency limits the rate of change in printhead-to-substrate velocity, including changes in direction. Also, the velocity of the drops moving from the printhead nozzles to the substrate declines with the distance traveled from the nozzles, and the paths of such drops become more greatly affected by air currents and other factors with increased nozzle to substrate distance. Additionally, droplet shape changes the farther the drop moves from the nozzle, which changes the effects of the drop on the substrate. Accordingly, variations in the distance from the printhead to the substrate can cause irregular effects on the printed image.
In addition to problems in jetting ink onto contoured surfaces, the curing of UV inks requires delivery of sufficient curing energy to the ink, which is often difficult to achieve where the surface is contoured.
Further, some substrates deform, even temporarily, when heated. Deformation caused by heat may be such that, for example, the material returns to its undeformed state when it cools. Nonetheless, even temporary deformation can adversely affect the print quality if it exists when ink is being jetted onto the substrate. Where spot curing of UV inks is employed, which is performed by exposing ink to UV immediately upon its contacting the substrate, UV that is accompanied by heat producing radiation can deform substrates such as foamboard while the ink jets are making single or multiple passes over the deformed print area.
For these reasons, ink jet printing has not been successful on contoured materials and other three-dimensional substrates, particularly when printing with UV curable inks.
An objective of the present invention is to provide for the ink jet printing onto substrates that tend to deform when heated. A particular objective of the present invention is to maintain desired printhead-to-substrate spacing when jetting ink onto rigid substrates, particularly with UV curable inks.
According to the principles of the present invention, printed images are applied to rigid substrates with printing elements that may be moveable relative to the plane of the substrate being printed. In certain embodiments, the invention provides a wide-substrate ink jet printing apparatus with printheads that move toward and away from the plane of a substrate to maintain a fixed distance between the nozzles of the printhead and the surface onto which the ink is being jetted. The variable distance over the plane of the substrate allows a controlled and uniform distance across which the ink is jetted.
According to the invention, the printing element may include an ink jet printhead set having a plurality of heads, typically four, each for dispensing one of a set of colors onto the substrate to form a multi-colored image. To maintain the constant distance or to otherwise control the distance, one or more sensors may be provided to measure the distance from the printhead or from the printhead carriage track to the point on the substrate on which ink is to be projected. Such sensors generate reference signals that are fed to a controller that controls a servo motor on the printhead carriage. The printhead may be moveably mounted to the carriage, for example, on a ball screw mechanism, and be moveable toward and away from the plane of the substrate by operation of the servo motor. Each printhead of the set may include four different color printheads that are separately moveable relative to a common printhead carriage, and are each connected to one of a set of four servo motors by which its position relative to the plane of the substrate is capable of control relative to the positions of the other printheads. The printheads of the set may be arranged side-by-side in the transverse direction on the carriage so that one head follows the other across the width of the substrate as the carriage scans transversely across the substrate.
Each printhead has, in the preferred embodiment, a plurality of ink jet nozzles thereon for dispensing a given color of ink in a corresponding plurality of dots, for example, 128 in number, that extend in a line transverse to the carriage, which is in a longitudinal direction perpendicular to the scan direction of the carriage. Two laser or optical sensors are provided on the carriage, one on each side of the heads, so that a distance measurement of the surface to the substrate can be taken ahead of the printheads when the heads are scanning in either direction. The controller records the contour of the substrate ahead of the printheads and varies the position of each printhead, toward and away from the substrate plane, as each printhead passes over the points at which the measurements were taken, so that each of the independently moveable heads follows the contour and maintains a fixed distance from the surface being printed. While it is preferred to adjust the position of the printhead or nozzle thereof relative to the substrate which is fixed on a printing machine frame, the substrate surface can alternatively be positioned relative to a printhead that is maintained at a fixed vertical position on the frame.
According to the preferred embodiment of the invention, UV ink is printed onto material and the cure of the ink is initiated by exposure to UV light radiated from UV curing lights mounted on the printhead carriage, one on each side of the printhead set. The lights are alternatively energized, depending on the direction of motion of the carriage across the substrate, so as to expose the printed surface immediately behind the heads. By so mounting the UV curing lights on the printhead carriage, the jetted ink can xe2x80x9cspot curexe2x80x9d the ink, or to cure the ink immediately upon its contacting the substrate. Such spot curing xe2x80x9cfreezes the dotsxe2x80x9d in position and prevents their spreading on or wicking into or otherwise moving on the substrate. With certain substrates, conventional or broad spectrum UV curing lights include radiation that can heat the substrate. Such radiation includes infra-red radiation and radiation of such other wavelengths that tend to heat a particular substrate.
In the case of many rigid substrates, such as foamboard and several other of the more commonly used substrates, energy radiating from the UV light curing source onto the substrate heats the substrate enough to deform it. Such deformation can deform rapidly, with the surface of the substrate rising or rippling within seconds of exposure. Usually, this deposition is temporary in that the substrate blisters or swells when heated but returns to its original condition immediately upon cooling. Where the UV exposure is carried out downstream of the printhead carriage, usually no harm results.
In the case of spot curing, the UV exposure occurs close to the point of printing. Deformation of the substrate surface that occurs due to heat in spot curing can extend to the portion of the substrate that is still to be printed, thereby changing the printhead-to-substrate spacing and adversely affecting the quality of the ink jet printing operation.
The present invention provides the use of cold UV sources for spot curing of UV curable ink on heat sensitive rigid substrates. Heat caused deformation of the substrate in the region of the printing operation is prevented with the use of a cold UV source, Such a cold UV source can, for example, be a limited bandwidth UV source, to limit energy of wavelengths that are not effective to cure the ink from otherwise striking and heating the substrate. This can be carried out with selective bandwidth sources or with the use of filters to remove energy of undesired wavelengths. Alternatively, heat removal can be employed to remove the heat that is produced by the curing radiation. The cold UV source is useful for printing onto substrates that can deform, even temporarily, when heated, and is particularly useful where spot curing of the ink can otherwise result in the deformation of the material on which printing is still to take place.
Deformation at the printing site, even if temporary such that the material returns to its undeformed state when it cools, adversely affects the print quality because spot curing deforms the substrate as the ink jets are making single or multiple passes over the print area. This is particularly the case when printing onto foamboards that make up the largest application of printing onto rigid substrates. Such deformation of the board from heat during printing would force adjustment of the head height above the deformation zone. Higher head height usually results in poorer print quality. With a cold-UV spot-cure ink-jet system, the head-to-substrate distance can be minimized to maximize print quality.
In prior practice, spot curing has not been used to ink jet print onto rigid substrates, except as proposed by applicants. Cold UV is known for curing UV ink downstream of a printing station to prevent permanent deformation to or buring of the substrate. Temporary deformation that will disappear after the substrate cools has not been a problem in the prior art. Such deformation is likely to be a problem where slight raising or warping of the surface takes place as ink is being jetted onto the substrate, which can occur during spot curing.
When printing onto contoured material, the distance from the printheads to the substrate where the ink is to be deposited can be determined by measuring the distance from a sensor to the substrate ahead of the printheads and mapping the location of the surface. For bidirectional printheads that move transversely across the longitudinally advancing fabric, providing two distance measuring sensors, one on each of the opposite sides of the printheads, are provided to measure the distance to the contoured fabric surface when the printheads are moving in either direction. For some inks and for sufficiently rigid materials, a mechanical rolling sensor may be used, for example, by providing a pair of rollers, with one roller ahead of, and one head behind, the printhead so that the average distance between the two rollers and a reference point on the printhead can be used to control the distance of the printhead from the plane of the substrate. To achieve this, one or more printheads can be mounted to a carriage having the rollers on the ends thereof so that the mechanical link between the rollers moves the printhead relative to the plane of the substrate. In most cases, a non-contact sensor, such as a laser or photo eye sensor, is preferred in lieu of each roller. The outputs of two sensors on opposite sides of the printheads can be communicated to a processor, to measure the distance from the heads to the fabric ahead of the bidirectional heads, to drive a servo motor connected to the printhead to raise and lower the head relative to the substrate plane so that the printheads move parallel to the contoured surface and jet ink onto the fabric across a fixed distance.
These and other objects of the present invention will be more readily apparent from the following detailed description of the preferred embodiments of the invention.